A bubble of 15? 4 days per week? Wouldn't it normally be more kids 5 days a week? Maybe not. We're any kids left out?
I will assume that you are utterly ignorant, rather than being wilfully obtuse.
Were you unaware that when the government authorised some year groups to return to school, this HAD to be in class 'bubbles' of no more than 15 (along with a whole raft of other safety arrangements)?
Each 'bubble' had its own teacher, and no staff were shared, again in line with covid guidelines in force at the time.
Every teacher is entitled to half a day of planning / preparation / assessment [non contact] time each week, and that could not be covered by another member of staff. Even if one could have been shared, every single member of my school's staff, except those shielding, were in the classroom during partial return, and every classroom was used to the maximum allowed.
The decision was therefore taken - since i was still also teaching my own class of 32 remotely - that the school would shut to the returned pupils for 1 day a week, to allow for full cleaning, PPA time and time for every teacher in with a 'not usual' year group to work with their own class.